1. Technical Field
The present invention relates to a seal member for a heating furnace. The present application claims priority from a Japanese Patent Application No. 2009-34062 filed on Feb. 17, 2009, the contents of which are incorporated herein by reference.
2. Related Art
An optical fiber is formed by suspending a quartz glass rod called an optical fiber perform rod (hereinafter simply referred to as “preform”) in a heating furnace to melt one end thereof, and drawing it. This process is referred to as “drawing.” The temperature of the heating furnace during the drawing process reaches around 2000 degrees centigrade. The heater is made of carbon, which is oxidized in the oxygen including atmosphere of a high temperature, and so an atmosphere having inert gasses such as Ar and He is maintained inside the heater.
In the drawing process, one end of the preform is suspended in the inert gas atmosphere, while the other end thereof is placed outside the heating furnace. So as to maintain the inert gas atmosphere, it is required to seal between the wall of the furnace and the surface of the optical fiber preform in an airtight manner. However, the manufacturing process of the optical fiber preform includes a sintering process such as VAD and OVD, which tends to cause different balances in longitudinal direction between the gravity exerted on a softened/melted section and the constriction force on a porous glass to be sintered, and this often causes the resulting optical fiber preforms to have outer diameter variations in the longitudinal direction.
Patent Document No. 1 describes a method of controlling the sintering condition by monitoring an amount of expansion/contraction of a preform when sintered. However, this method necessitates adjusting the outer diameter of the core of the preform in the longitudinal direction in advance, taking into consideration the expansion and contraction thereof. In addition, in applying soot around the core rod, this method requires adjusting the amount of soot to correspond to the core rod diameter in the longitudinal direction, which involves substantial man-hours.
Furthermore, even when the drawing process is performed after elongating, into a thin diameter, a perform with an adjusted outer diameter after depositing soot and sintering it, large variation still tends to be caused in the resulting outer diameter of the preform, especially at the start and the end of the elongated preform. Such portions of the preform cannot be subjected to the subsequent drawing process, and so should be discarded. Therefore, it is required to provide a method for effectively sealing the space between a heating furnace and a preform, even for processing preforms having different outer diameters from each other.
Patent Document No. 2 describes a seal ring which is a combination of an inner seal ring and an outer seal ring, each of which is made of a plurality of parts constituting a ring. This seal ring is pressed against the surface of the preform by means of a return spring outside of the outer seal ring.
However, according to this structure, the ring-constituting parts and their peripheral parts should be processed very accurately, so that each ring-constituting part operate evenly according to the outer diameter adjustment. Moreover, the ring-constituting parts of the ring should have a certain height to cooperate each other when moving.
When providing the seal rings in many layers to improve airtightness, the weight of the seal rings will be considerable. In the drawing process, the preform is moved in the horizontal direction, so as to adjust the optical fiber to the center of the diameter measuring apparatus. In such a drawing process, when the preform is heavy, the seal ring does not follow smoothly, and the amount and direction of the movement of it becomes difficult.
Patent Document No. 3 discloses a sealing method using a doughnut-shaped brush made of carbon optical fiber. This method enables to obtain a very stable, favorable, and light-weight seal which does not necessitating such an accurate process as described above. However, a drawback is that the carbon optical fiber, which makes up the brush, occasionally fell off, to be adhered to the surface of the preform to reduce the strength of the optical fiber.
Patent Document No. 4 relates to a seal member which the inventors of the present application have proposed previously. The seal member is made of a heat resistant elastic member, and is made by laminating center-hollow disks, each of which has a plurality of incisions radially from the central aperture. The seal member is provided at the top of the heating furnace, specifically at the port for the preform to come in. This seal member has such a structure that, in lowering a rod, the inner surface of the doughnut-shaped disk structure makes a smooth curve towards the insertion direction of the rod to move along the surface of the rod, thereby favorably sealing the gas inside the heating furnace.
However, in raising the rod, the inner edge of this seal member is occasionally stuck on the rod surface, to leave a space on the surface of the rod. In addition, sometimes this seal member is bent. If the seal member is bent, the sealed state of the gas in the furnace may change to accordingly change the flow of the gas in the furnace. Moreover, sometimes this seal is broken, to let the outer air come in the heating furnace.
The following lists the patent documents used in this specification.    Patent Document No. 1: Japanese Patent Application Publication No. 2005-8452    Patent Document No. 2: Japanese Patent Application Publication No. 2006-342030    Patent Document No. 3: Japanese Patent Application Publication No. 2007-70189    Patent Document No. 4: Japanese Patent Application No. 2007-191694